Capacitors vs inductors and current vs voltage.

[Apokalypz]

I've had a good understanding of what a capacitor does since I was a kid. It charges up to the line voltage (in the he case of a decoupling or filter cap) and when the line voltage dips, it releases some of its voltage... Now I know there is one very important mistake at the end of that sentence, but bare with me.

Then I learned what an inductor does a few years ago. It stores current in a magnetic field and releases the current when the current in the circuit drops... Again, I realize the mistake (similar to the mistake I made above).

Now, after really thinking about it, it hit me and I discovered where my understanding is flawed. It all revolves around Ohm's law. A capacitor doesn't release it's "voltage", that actually doesn't make sense, it actually releases current to maintain the voltage through the load. Again V=IR. Same thing goes for the inductor, it pushes a higher voltage through the load to maintain the current.

I think there's a problem when people try to explain what these passive components do to beginners in EE. It seems like people always use voltage to describe what a capacitor does and always use amperage to describe what an inductor does. Nobody really talks about how the components react with the load to give the complete picture. You just don't think about current when talking about capacitors and voltage when talking about inductors.

So I rewrote my own understanding of each component as follows:
A capacitor tries to maintain voltage by supplying a variable amount of current to the circuit.
An inductor tries to maintain current by pushing a variable voltage through the circuit.

So my question is, does my new understanding make more sense? What do you guys think?

[IanB]

Your rewritten sentences are approximately fine.

Another way of saying them is this:

"A capacitor doesn't like to change its voltage"
"An inductor doesn't like to change its current"

A capacitor "reacts" to attempts to change its voltage by sourcing or sinking current.
Similarly an inductor "reacts" to attempts to change its current by increasing or decreasing the voltage across its terminals.

This is why such components are said to have reactance in a circuit (rather than resistance).

[Apokalypz]

"A capacitor doesn't like to change its voltage"
"An inductor doesn't like to change its current"

This is more or less the explanation given in textbooks and tutorials and is how I came to understand the components when I learned about them. I'm just surprised that all the explanation I've seen limit themselves to this understanding while it doesn't really say how caps maintain voltage or how inductors maintain current.

Thanks for the reply.

[Ratch]

I've had a good understanding of what a capacitor does since I was a kid. It charges up to the line voltage (in the he case of a decoupling or filter cap) and when the line voltage dips, it releases some of its voltage... Now I know there is one very important mistake at the end of that sentence, but bare bear with me.

A cap does not "charge up."  It contains the same amount of net charge at 10, 100, or 1000 volts as it does at zero volts.  For every coulomb of charge imposed on either plate, the same amount of charge is removed from the opposite plate. This means that the net loss/gain of charge is zero.  What you should understand is a cap stores energy in the form of an electrostatic field.  Everyone should say a capacitor becomes energized, not charged.  A cap energized to 100 volts contains more energy that a cap energized to 10 volts (E = 1/2 C V^2).

Then I learned what an inductor does a few years ago. It stores current in a magnetic field and releases the current when the current in the circuit drops... Again, I realize the mistake (similar to the mistake I made above).

Wrong!  Neither a cap or coil store either voltage or current.  Both store electrical energy (electric fields or magnetic fields).  Both the voltage and current involved are consequences of how fast the energy is stored and released, and what type of electrical energy (electric or magnetic) fields are involved.

Now, after really thinking about it, it hit me and I discovered where my understanding is flawed. It all revolves around Ohm's law. A capacitor doesn't release it's "voltage", that actually doesn't make sense, it actually releases current to maintain the voltage through the load. Again V=IR. Same thing goes for the inductor, it pushes a higher voltage through the load to maintain the current.

Same answer as above.  Voltage is the energy density of a charge (joules/coulomb).  A density (voltage) cannot be stored.  It just exists.  You would not say velocity can be stored, would you?  Neither can density. If a voltage exists across the plates of a cap, you can be sure that energy is being stored in the cap.  The voltage represents the electric field energy with respect to the amount of charge difference between the plates (joules/coulomb).

I think there's a problem when people try to explain what these passive components do to beginners in EE. It seems like people always use voltage to describe what a capacitor does and always use amperage to describe what an inductor does. Nobody really talks about how the components react with the load to give the complete picture. You just don't think about current when talking about capacitors and voltage when talking about inductors.

You have to understand how the speed those electric energy storage components accept and release energy affects the voltage and current.

So I rewrote my own understanding of each component as follows:
A capacitor tries to maintain voltage by supplying a variable amount of current to the circuit.
An inductor tries to maintain current by pushing a variable voltage through the circuit.

Wrong.  The voltage and current are due to the rate of build-up and collapse of the electric and magnetic fields.

So my question is, does my new understanding make more sense? What do you guys think?

You are wrapping yourself around the axle.  You had better reply to this thread with more questions and examples.

Ratch

[IanB]

"A capacitor doesn't like to change its voltage"
"An inductor doesn't like to change its current"

This is more or less the explanation given in textbooks and tutorials and is how I came to understand the components when I learned about them. I'm just surprised that all the explanation I've seen limit themselves to this understanding while it doesn't really say how caps maintain voltage or how inductors maintain current.

I was going to reply earlier today but I didn't have a chance.

However, the answer to your question is that a change in the voltage on a capacitor or a change in the current in an inductor involves an change in stored energy, which put another way means thermodynamic work has to be done for the voltage or current to change. The need for work to be done on or by the component is what makes the voltage or current resist change.

For a mechanical analogy you can consider a heavy flywheel. It takes an effort to get the flywheel spinning, and once it is spinning takes an effort to slow it down. This is because the spinning flywheel is stores energy. You have to do work to speed it up, and it will do work on the surroundings if you try to slow it down again.

[Ratch]

So what does "electrify" mean?  Just about anything you want it to mean as long as it pertains to something involving electromagnetic science.  To say that a capacitor becomes energized describes precisely what is happening.  It is storing energy.  Why talk technical trash TTT) like NASA does when they describe their astros as "walking in space".   When they work outside their capsule, do the look like they are walking? If their tether should break, would they walk away?  It is all right to use colloquial expressions for informal banter.  But when precise meanings and technical descriptions must be conveyed, then precise language should be used.

Ratch

[IanB]

But when precise meanings and technical descriptions must be conveyed, then precise language should be used.

Precise and useful. To say a capacitor stores no charge when it is charged up is a precisely pointless statement. It has no value and serves no purpose.

[Brumby]

But when precise meanings and technical descriptions must be conveyed, then precise language should be used.

The language is precise.

When someone says a capacitor is "charged" - everybody who is familiar with the concept knows exactly what is meant.  It is unambiguous.  It is precise.

It is only those who do not understand it who will trip over any potential confusion in the use of the words "charge" and "charged".  This is where education is required.

It is unfortunate that we feel the need to find an alternate word to the one that is in common usage across an industry, just to address the issue of two words spelled very similarly.

[Ratch]

But when precise meanings and technical descriptions must be conveyed, then precise language should be used.

Precise and useful. To say a capacitor stores no charge when it is charged up is a precisely pointless statement. It has no value and serves no purpose.

So why do folks say a cap is charged when it is not?  Unless you mean charged with energy.  Then you might as well say energized.   Actually, what I said was a cap stores a charge on one plate and depletes the same charge on the opposite plate for a net change of zero.

Ratch

[Brumby]

But when precise meanings and technical descriptions must be conveyed, then precise language should be used.

Precise and useful. To say a capacitor stores no charge when it is charged up is a precisely pointless statement. It has no value and serves no purpose.

So why do folks say a cap is charged when it is not?  Unless you mean charged with energy.

This:

When someone says a capacitor is "charged" - everybody who is familiar with the concept knows exactly what is meant.  It is unambiguous.

You are focussing on:

... just to address the issue of two words spelled very similarly.

[Ratch]

But when precise meanings and technical descriptions must be conveyed, then precise language should be used.

The language is precise.

When someone says a capacitor is "charged" - everybody who is familiar with the concept knows exactly what is meant.  It is unambiguous.  It is precise.

It is only those who do not understand it who will trip over any potential confusion in the use of the words "charge" and "charged".  This is where education is required.

It is unfortunate that we feel the need to find an alternate word to the one that is in common usage across an industry, just to address the issue of two words spelled very similarly.

The language is syntactically correct, but the meaning is wrong.  Its correct meaning is well understood by a clique of insiders who have practiced electrical science, and used the wrong phraseology so long that they do not give it any thought. 

Another bogus phrase is "current flow" which literally means "charge flow flow".  Charge doesn't flow twice.  Folks should just say "current exists", or current is present, or just current if it is grammatically correct.

Ratch

[Brumby]

Be the pedant all you want - but the terms are well understood, despite any real or perceived tautological aberrations.

[Brumby]

To show how silly this type of debate can be ....

If we take your assertion at face value, then you can never say the current is 0A.  You can't talk about the flow of charge if there is no charge flowing.  The term "current" becomes undefined at that point.

[Ratch]

Be the pedant all you want - but the terms are well understood, despite any real or perceived tautological aberrations.

I believe talking technical trash (TTT) confuses newcomers and makes it harder to learn the basics.

Ratch

[Brumby]

Be the pedant all you want - but the terms are well understood, despite any real or perceived tautological aberrations.

I believe talking technical trash (TTT) confuses newcomers and makes it harder to learn the basics.

Ratch

... and you think this sort of thing is helpful?

I can see your point - but I don't believe it's the best way to go.

[Ratch]

To show how silly this type of debate can be ....

If we take your assertion at face value, then you can never say the current is 0A.  You can't talk about the flow of charge if there is no charge flowing.  The term "current" becomes undefined at that point.

I don't understand what you are saying.  Why can't we say no current exists, or no current is present?  Amps are the rate of charge flow.  Why can't zero amps mean a rate of zero for charge flow?  Why can't a vehicle have a zero velocity or zero rate of movement when stopped?  Inquiring minds would like to know.

Ratch

[Brumby]

Another bogus phrase is "current flow" which literally means "charge flow flow".  Charge doesn't flow twice.  Folks should just say "current exists", or current is present, or just current if it is grammatically correct.

The only thing bogus is your understanding of the English language.

"current(noun) flow(verb)"
"charge flow(noun) flow(verb)"

The use of the word "flow" is different - and therefore, legitimate.

To argue that it is repetitive and inappropriate shows a lack of understanding of grammatical structure and a focus on the arrangement of individual letters.

[Ratch]

Be the pedant all you want - but the terms are well understood, despite any real or perceived tautological aberrations.

I believe talking technical trash (TTT) confuses newcomers and makes it harder to learn the basics.

Ratch

... and you think this sort of thing is helpful?

I can see your point - but I don't believe it's the best way to go.

I am in agreement with a lot of folks who believe that it is best to learn it correct from the beginning and not use sloppy definitions or concepts.

Ratch

[Brumby]

Why can't a vehicle have a zero velocity or zero rate of movement when stopped?

We can - but that example doesn't have the same grammatical error that your assertion about "current flow" does.

[Brumby]

I am in agreement with a lot of folks who believe that it is best to learn it correct from the beginning and not use sloppy definitions or concepts.

Ratch

So - Do you promote the teaching of electron flow rather than conventional current flow?  (Oh, darn ... there's that phrase again!)

[Brumby]

This is getting tedious and it's ground that's been covered, trampled and stomped on before ... a number of times.

One more response here - and then I'm done with this thread.

[Ratch]

Another bogus phrase is "current flow" which literally means "charge flow flow".  Charge doesn't flow twice.  Folks should just say "current exists", or current is present, or just current if it is grammatically correct.

The only thing bogus is your understanding of the English language.

"current(noun) flow(verb)"

Ratch
"charge flow(noun) flow(verb)"

The use of the word "flow" is different - and therefore, legitimate.

To argue that it is repetitive and inappropriate shows a lack of understanding of grammatical structure and a focus on the arrangement of individual letters.

No, "current" in electrical science means charge flow or rate of charge movement.  If you want to use rate as a noun, then it implies charge movement, and the second word "flow" is redundant and ridiculous.  Find any sentence using current flow and see how much better it sounds using my suggestion.

[Ratch]

Why can't a vehicle have a zero velocity or zero rate of movement when stopped?

We can - but that example doesn't have the same grammatical error that your assertion about "current flow" does.

I don't think you proved my assertion about current flow is wrong.  See my answer to your statement.

Ratch

[Ratch]

I am in agreement with a lot of folks who believe that it is best to learn it correct from the beginning and not use sloppy definitions or concepts.

Ratch

So - Do you promote the teaching of electron flow rather than conventional current flow?  (Oh, darn ... there's that phrase again!)

Yes, you should have left off the word flow in the last sentence.

I always use the mathematical convention that positive charges flow from the positive terminal and return to the negative terminal of a voltage source.  That is the way semiconductors are marked and ammeter polarities are shown.  That does not always agree with the real physical direction of the charges, but it is easy to reverse direction if the real physical direction of negative charges need to be known.  Assigning a physical direction at the beginning of an analysis by the polarity of the charge carriers is a recipe for confusion.

Ratch

[IanB]

Thank you for derailing yet another thread and filling it with off-topic posts.